Etiology of Disease

Historical Background

Investigations of disease etiology in the modern period of epidemiology have continued to evolve with the dominant paradigms of disease causation of each era. The sanitary era of the early 19th century was dominated by the miasma theory, a view of disease as arising from foul emanations from impure water, soil, and air. English sanitary reformers who held to this theory, such as Edwin Chadwick and Florence Nightingale, were thus mistaken in their understanding of the specific agents of disease etiology; however, their overall emphasis on improving social and physical conditions in the urban industrial environment led to major improvements in public health.

The work of Louis Pasteur and the development of Koch's postulates next contributed to the shift in clinical medicine and public health toward the germ theory, which attributes disease causation to microorganisms. During the ensuing era of infectious disease of the late 19th and early 20th centuries, etiologic research accordingly consisted of searching for single causative agents of diseases. The classic model that reflects the influence of this era of infectious diseases is the epidemiologic triad of agent, host, and environment.

By the mid-20th century, a shift had occurred in more developed countries, with chronic rather than infectious diseases becoming predominant. Etiologic research in the era of chronic disease epidemiology focused on multiple proximate risk factors for chronic diseases in individuals and benefited from advances in epidemiological and biostatistical methods.

The latter part of the 20th century witnessed increasing developments within epidemiology and related fields that have contributed depth and breadth to the field on both a microand macrolevel. Advances in the subdisciplines of molecular epidemiology and human genome epidemiology enabled epidemiologists to use findings from the Human Genome Project and to study the specific pathways, molecules, and genes, and interactions between genes and the environment that [Page 351]influence the risk of developing disease. Concomitantly, a renewed focus on the effects of the social and physical environments on health and an increasing interest in intergenerational influences were reflected in the development of a life course approach to chronic disease epidemiology and the contributions of social epidemiology to a multilevel perspective of disease etiology.